Congress has until Tuesday to agree on funding for federal agencies in order to avoid a partial government shutdown. So let's look this morning at exactly what that shutdown would mean.

INSKEEP: Lawmakers are coming near the wire because of Republican efforts to include a provision to defund the Affordable Care Act, also known as ObamaCare. Now, if a shutdown were to occur, a big question is which federal employees would stay home and which ones would be deemed essential.

NPR's Brian Naylor has more.

BRIAN NAYLOR, BYLINE: So let's say you were planning on a weekend getaway to Acadia National Park next month or an anniversary trip to Italy and you need a passport. Well, if the government shuts down, you're going to need another plan. Acadia and all of the nation's 401 National Parks will be closed, and passport offices will be shuttered. They're part of the government that relies on annual appropriations bills, none of which Congress has approved to fund their services. The IRS, Department of Education and various regulatory agencies would all be closed.

On the other hand, if you're counting on a Social Security check, food stamps or unemployment insurance, you'll be OK. That's because those programs are more or less on auto-pilot and don't rely on annual appropriations bills to keep operating. And there are other programs exempt from a shutdown. Active duty military, Border Patrol, meat inspectors and air traffic controllers - those jobs are considered essential.

Colleen Kelley, who heads the National Treasury Employees Union, says there is a pretty clear line between those agencies protected from a shutdown and those that are not.

COLLEEN KELLEY: So in an agency like Homeland Security, an awful lot of the employees would continue to work because they protect every port of entry into our country. But in most agencies that are not involved with national security, like the IRS, my expectation is that the majority of them will not be working.

NAYLOR: And that includes thousands of civilian Defense Department employees who, the Pentagon says, would be temporarily furloughed.

All told, more than 800,000 federal employees will be idled by a shutdown, not counting the large number of contractors who won't be paid. After the last big government shutdown in 1995 and '96, the Congressional Budget Office estimated the cost at $1.4 billion.

But political science professor Roy Meyers of the University of Maryland says it's hard to estimate the true cost of a shutdown.

ROY MEYERS: For example, what does it cost the American people when you tell somebody who leads the Centers for Disease Control and Prevention to plan for a shutdown rather than try to reduce or eliminate public health threats? To me the cost there is the increased risk associated with not paying as much attention to public health.

NAYLOR: Members of Congress themselves, despite what some might think, are considered essential and will be on the job, though most of their staff members are likely to be told to stay home. Meyers says the time lawmakers spend bickering over the shutdown is time they're not spending on other issues.

MEYERS: Think, for example, about immigration. After the last presidential election, most people in Washington say, well, this is the year to have an immigration law. Where is it? Well, there are still big issues out there to resolve. And one reason why they're not resolved is that Congress is spending far too much time fighting over this stupid little shutdown idea.

NAYLOR: The most affected by the shutdown will be the federal workers who are deemed non-essential. Kelley, of the Treasury Employees Union, says the threat of a shutdown has federal employees fearful and anxious.

KELLEY: Federal employees are in their third year of a pay freeze. And many have served unpaid furlough days in the last six months or so. So the idea that they would suffer additional economic impact by a government shutdown, it's very disheartening for them.

NAYLOR: And it's unclear if federal workers will get paid for the time they miss if there's a shutdown. They were the last time, but it takes an act of Congress. And it's far from certain whether lawmakers this time around will be sympathetic to the employees' plight.